1. Field of the Invention
The present invention relates to a digital communication system using phase modulation and, more particularly, to a phase modulation scheme and apparatus.
2. Description of the Prior Art
FIG. 1 shows a conventional phase modulation apparatus. Referring to FIG. 1, the phase modulation apparatus is constituted by a bandwidth limiting filter 12 which receives two transmission baseband signals I.sub.IN (t) and Q.sub.IN (t) and performs band limitation by removing high-frequency components, and a phase modulator 13 which receives outputs I.sub.F (t) and Q.sub.F (t) from the bandwidth limiting filter 12 and outputs a phase-modulated signal S(t). In this phase modulation apparatus, the outputs I.sub.F (t) and Q.sub.F (t) from the bandwidth limiting filter 12 are always phase-shifted by .pi. [rad], as shown in FIG. 2. FIGS. 3 and 4 show the transmission simulation results of BPSK modulation in which the two outputs are phase-modulated by the phase modulator 13. In the BPSK modulation, the two transmission baseband signals I.sub.IN (t) and Q.sub.IN (t) are identical. FIG. 3 shows a case wherein the BPSK-modulated wave which has undergone band limitation is amplified by a nonlinear amplifier. FIG. 4 shows a case wherein the BPSK-modulated wave is amplified using the saturation region of a class C amplifier.
As described above, in the conventional BPSK modulation apparatus, the output from the modulator is always phase-shifted by .pi. [rad] in accordance with a change in signal state transition of the transmission baseband signal. Although not described above, a QPSK modulator also has a state transition mode with a phase shift of .pi. [rad]. When the phase-modulated wave which has undergone band limitation is amplified by the nonlinear amplifier, the spectrum extends beyond the estimated transmission bandwidth to cause spectral spillover, and spurious noise or envelope variations increase due to the conversion distortion of the amplifier, as shown in FIG. 3. This also applies to amplification using the class C amplifier, as shown in FIG. 4. For this reason, interference from adjacent channels poses a problem. Additionally, the conversion distortion increases phase errors in the transmission signal. These problems increase the digital error rate on the receiver side.
To avoid such problems, the conventional BPSK modulation uses ABPSK (.pi./2-shift BPSK). In QPSK modulation, .pi./4-shift QPSK or OQPSK is used. However, since ABPSK, .pi./4-shift QPSK, and OQPSK require corresponding demodulation techniques, the characteristics of the modulated wave are not improved in a communication system using BPSK or QPSK modulation.